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Water-soluble organic compounds have recently received much attention because of their ability to absorb water and alter the hygroscopic properties of inorganic aerosols. The effects of glycerol, succinic acid, malonic acid, citric acid, and glutaric acid on the water cycles (water activities during particle evaporation and growth), crystallization relative humidities (CRH), and deliquescence relative humidities (DRH) of sodium chloride (NaCl) and ammonium sulfate (AS) were studied using an electrodynamic balance (EDB). The growth factors of these inorganic and organic mixtures were lower than those of the pure inorganic species. The presence of all these organics in the mixed particle reduce the water absorption of NaCl but enhance that of AS relative to that of the pure inorganic salts. Glycerol and succinic acid did not affect the deliquescence properties of NaCl and AS, although succinic acid increased the CRH of NaCl and AS. Malonic acid and citric acid, behaving as nondeliquescent species in single particle studies, caused NaCl and AS particles to absorb a significant amount of water before deliquescence. Glutaric acid caused NaCl and AS to deliquesce gradually, spanning a wide range of relative humidity. The ZSR model was found to be useful in predicting the water activity of the mixtures and the growth ratios. However, the detailed crystallization and deliquescence behaviors of the organic/inorganic mixtures cannot be easily predicted from the hygroscopic properties of the individual components.

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Salts of the same anions of Mg2+, Na+, and Li+ have been found to exhibit different hygroscopic properties. These differences are attributed to the molecular structural properties of the hydrogen bonding network of the water molecules in the second and first hydrated layers of Mg2+, Na+, and Li+. To study the structures of water molecules, in particular, the presence of water monomers, Raman spectra of single levitated droplets of aqueous NaClO4 , LiClO4, and Mg(ClO4)2 solutions from diluted concentrations to high supersaturations were measured. Because heterogeneous nucleation was suppressed in these levitated droplets, supersaturated droplets with a water-to-solute ratio (WSR) as low as 2 was achieved. Taking advantage of the structure breaking effect of ClO4 - on the hydrogen bonding network of water molecules, Raman spectra of water monomers in these highly supersaturated droplets were observed. At a low WSR, two peaks at 3549 and 3588 cm-1 for water monomers with one and two weak hydrogen bonds with ClO4 - were observed for NaClO4 droplets. Compared with those of the NaClO4 droplets, the peaks of the water monomers in supersaturated LiClO4 and Mg(ClO4)2 droplets red-shifted to 3553 and 3546 cm-1, respectively. This observation is consistent with the order of the increase of the polarization effect of the cations, i.e., Na+ < Li+ < Mg2+. The intensity ratios of the strongly hydrogen-bonded components to the water monomers, i.e., I 3440/I 3549 and I 3289/I 3549 for NaClO4, I 3455/I 3553 and I 3275/I 3553 for LiClO4, and I 3411/I 3546 and I 3440/I 3549 for Mg(ClO4)2, were used to study the effects of the presence of ClO4 - on the water structures of the hydration layers of Na+, Li+, and Mg2+. The results were explained in terms of the stability of water molecules in the inner spheres of these hydrated cations.

Significant retardation of the evaporation rate of levitated aqueous MgSO4 droplets has been found at high concentrations using an electrodynamic balance. Raman spectroscopy was used to study the structural changes, in particular, the formation of contact ion pairs, in supersaturated aqueous MgSO4 droplets at ambient temperatures. As the relative humidity (RH) decreases, single levitated droplets lose water and become supersaturated. A molar water-to-solute ratio as low as 1.54 was obtained, facilitating the study of contact ion pairs of unhydrated Mg2+ and SO4 2- ions in MgSO4 solutions. The characteristics of the ν1-SO4 2- band change as a function of the water-to-solute ratio. Overall, a frequency shift from 983 to 1007 cm-1 and an increase of the full width at half-height from 12 to 54 cm-1 of the ν1-SO4 2- band were observed when the water-to-solute molar ratio decreased from 17.29 to 1.54. Most of the changes occur at a ratio smaller than 6, instead of at the saturation ratio of 15.60. These changes are attributed to the formation of contact ion pairs with different structures. A chain structure based on the contact ion pair of bidentate was proposed. Formation of close contact ion pairs and chain structures can explain the retardation of evaporation of supersaturated MgSO4 droplets observed in previous experiments. On the basis of the comparisons of the Raman spectra of MgSO4 and (NH4)2SO4, we assigned the shoulder appearing at 995 cm-1 in bulk MgSO4 studies to the contact ion pairs instead of the solvent-separated ion pairs.

Water-soluble organic compounds have recently received much attention because of their ability to absorb water and affect the radiation balance and the climate. Partly because of their relatively high volatility, thermodynamic data on water-soluble organic compounds are scarce. Recently, we have developed a method based on the scanning electrodynamic balance (SEDB) that enables the measurement of water activity data of evaporating droplets within an hour, which can potentially be used to measure volatile species. This paper demonstrates the use of the SEDB to study the hygroscopic growth of selected atmospheric species, including semivolatile organic species with vapor pressure up to 1 × 10 -4 mmHg. We also measured the water activities, the crystallization relative humidity, and the deliquescence relative humidity (DRH) of aqueous solutions of maleic acid and glutaric acid. The DRHs of maleic acid and glutaric acid are in general agreement with the literature, except that glutaric acid shows a small delay in the completion of deliquescence due to masstransfer limitation. The water activities of equal molar mixtures of maleic acid and malic acid and of malonic acid and glutaric acid were also measured. The Zdanovskii-Stokes-Robinson (ZSR) predictions agree well with the measurements of the mixtures. The UNIFAC (UNIQUAC functional group activity coefficients) predictions, using the modified functional group interaction parameters of COOH, OH, and H 2 O derived from our earlier measurements of the water activities of aqueous droplets of a list of dicarboxylic and multifunctional acids, are also in agreement with the mixture data.

Heterogeneous oxidation of SO2 is one of the promising mechanisms to account for high loading of sulfate during severe haze periods in China. Our earlier work reported on the SO2 oxidation by OH and NO2 produced during 250 nm nitrate photolysis (Environ. Sci. Technol. Lett. 2019, 6, 86–91). Here, we extend that work to examine sulfate production during nitrate photolysis at 300 nm irradiation, which can additionally generate NO2 – or HNO2, N­(III). Flow cell/in situ Raman experiments showed that the reactive uptake coefficient of SO2, γSO2 , can be expressed as γSO2 = 1.64 × p NO3–, where p NO3− is the nitrate photolysis rate in the range of (1.0–8.0) × 10–5 M s–1. Our kinetic model with the p NO3− predicts that N­(III) is the main contributor to the SO2 oxidation, followed by NO2 contribution. Furthermore, the addition of OH scavengers (e.g., glyoxal or oxalic acid) does not suppress the sulfate production because of the reduced N­(III)-consuming reactions and the high particle pH sustained by their presence. Our calculations illustrate that under characteristic haze conditions, the nitrate photolysis mechanism can produce sulfate at ∼1 μg m–3 h–1 at pH 4–6 and p NO3– = 10–5 M s–1. The present study highlights the importance of in-particle nitrate photolysis in heterogeneous oxidation of SO2 by reactive nitrogen (NO2 –/HNO2 and NO2) under atmospherically relevant actinic irradiation. However, the nitrate photolysis rate constant needs to be better constrained for ambient aerosols.